Multiple input multiple output (MIMO) is an advanced antenna technique to improve the spectral efficiency and thereby boosting the overall system capacity. The MIMO technique uses a commonly known notation (M×N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (8×4). The configurations represented by (2×1) and (1×2) are special cases of MIMO and they correspond to transmit diversity and receiver diversity respectively. The configuration (2×2) is used in WCDMA release 7 and configurations ((4×4), (4×2) (4×1)) are being defined in Third Generation Partnership Project (3GPP) release 11.
Currently a 4Tx transmissions scheme for High-Speed Downlink Packet Access (HSDPA) is discussed within 3GPP standardization. Previous versions of the specification supported up to 2 TX antenna transmissions. In order to support 4Tx MIMO transmissions, it is necessary to obtain 4 channel estimates in order to characterize each of the spatial layers, which will require new pilot signals to be defined. Pilots are needed for two main functionalities; channel state information (CSI) estimation through channel sounding where rank, a channel quality indicator (CQI) (also referred to as channel quality information) and a precoding control index (PCI) are estimated and channel estimation for demodulation purposes.
For 4 branch MIMO, at least two different approaches are possible (1) Common pilots for both CSI and channel estimation for data demodulation and (2) Common pilots for CSI estimation and additional pilots for channel estimation for data demodulation.
In the above context, “common pilots” refer to pilots that are made available to all users and which are transmitted without user specific beamforming.
Common pilots may be transmitted at instances in which legacy users (Release 7 MIMO and Release 99), who are not able to demodulate 4TX transmissions are scheduled. These legacy users cannot make use of the energy in the common pilots. However, the energy in the additional common pilots will reduce the amount of energy available for High-Speed Physical Downlink Shared Channel (HS-PDSCH) scheduling to the legacy users. Moreover, the additional pilots cause interference to these users. Therefore, to minimize performance impacts to non 4TX users, it is essential that the power of the common pilots can be reduced to a low value.
In a High-Speed Downlink Packet Access (HSDPA) system, the user equipment (UE) (e.g., communication device) has to periodically report a channel quality indicator (CQI). The period is configured by the higher layers. For the purpose of channel quality reporting the UE has to know the powers of the pilots and the measurement offset (a.k.a., measurement power offset).
In Release 7 MIMO (2×2), for the purpose of CQI reporting, the UE shall assume a total transmit power of HS-PDSCH (PHSPDSCH) is given by:PHSPDSCH=PCPICH+Γ in dB,where the total transmit power is assumed to be evenly distributed among the HS-PDSCH codes corresponding to the reported CQI value, and the measurement power offset Γ (i.e., an offset value) is signaled by higher layers. PCPICH denotes the combined transmit power of a set of CPICH(s) (e.g., a primary CPICH and three secondary CPICHs) used for MIMO operation of the High-Speed Downlink Shared Channel (HS-DSCH).
The base station (e.g., Node B) sends the measurement power offset information through higher order signalling for example Radio Resource Control (RRC) messages. The actual measurement power offset can vary from TTI to TTI since HS-PDSCH power depends upon power transmitted by Node B when user is scheduled. A user can be scheduled every TTI. To reduce overhead, the measurement power offset is signalled preferably only at the start of an HSDPA session. Therefore, the Node B re-computes the actual CQI based on the current or recent measurement power offset and the signalled measurement power offset. The Node B knows the signalled measurement power offset. In a four branch MIMO, it was decided to use two types of pilot solutions. The Node B informs the UE which one of the two to use. In each case, the measurement power offset computation is different. Hence a new method to determine the measurement power offset and convey this information is desired.